The kinetics of nucleophilic substitution by sulfite ion of l'methyl thiaminium ion and several of its derivatives are to be determined. Reactions are to be carried out at 25 degrees C at 1.0 M ionic strength. Changes will be followed by a spectrophotometric method. The purpose of this work is to determine the extent and nature of leaving group effects on reactivity. This work with model substrates may provide information which may lead to an understanding of the mechanism of action of thiaminase I.